Rotary optical encoders are often used to measure the angular position of a motor shaft. Presently known conventional devices employ optical detectors to monitor the motion of a disk that is attached to the motor shaft. The optical detectors and an associated light source are mounted within read stations, or heads, which are fixed or locked in a stationary position with respect to the encoder housing. Typically the disk has a series of light and dark lines encoded on its surface which are illuminated in the region of the optical detectors by the light source. The portion of the disk thus demarcated is referred to as the track. As the illuminated disk rotates beneath the detectors, the amount of illumination impinging on their surfaces fluctuates. The amount of shaft rotation is determined by counting the number of intensity fluctuation sensed by the detector. Since the angular width of the lines is known at a particular radius on the disk, the arclength viewed by the head and the associated angular rotation of the disk may be determined.
There are fundamentally three optical arrangements which can be used. The most common is a transmissive scheme wherein opaque lines are encoded on a transparent disk. The light source is placed opposite the optical detectors with the disk rotating between the two. In order to enhance contrast, a mask between the disk and the detectors may be employed and collimating optics may be used.
A second approach is to place the detectors and the light source on the same side of the disk. In this reflective scheme, the disk is constructed such that it reflects varying amounts of light back to the detectors. A variation on this scheme involves applying the principles of interferometry. The disk is grooved such that the stripes on the disk lie in two planes differing by a fraction of a wavelength of light. A third approach is similar but is based upon principles of diffraction and interferometry. In this approach the disk is constructed such that it acts as a diffraction grating.
One limitation of conventional rotary encoders is their sensitivity to eccentricities in the disk or shaft relative to the detectors. These cause the radius from the center of rotation to the portion of the track being observed by the read stations to vary. In order to properly interpret the fluctuations in illumination in terms of arclength, one must have knowledge of the instantaneous radius throughout the sweep of the disk. Otherwise, the calculated or perceived rotation will deviate from the actual rotation. Given that the conventional encoder has fixed heads, this deviation cannot be accounted for by the individual heads. In order to minimize sensitivity to this phenomenon, multiple heads are often used and the detected signals are averaged. If, however, the encoder track being monitored should deviate to such an extent that it lies outside the detector's field of view, no motion of the disk will be detected. Thus, eccentricity has a significant effect on accuracy.
The encoder device disclosed herein minimizes the effects of eccentricity and other misalignments between the disk and the read stations by employing heads which incorporate beam steering optics with the ability to actively track the disk in directions along the disk radius and normal to its surface. The design of the device is such that it employs a reflective disk and the principles of interferometry. The servo controlled steering optics will move so as to acquire a track on the disk lying at a predetermined radius and distance below the head, and then adjust position and orientation in order to maintain view of the disk track as required. Thus, the device will be actively self-aligning.
The precision bearings used in conventional encoders to align the disk relative to the read stations are no longer necessary. Consequently, the costs associated with precision assembly are reduced. The load path through the bearings is eliminated, thereby preventing loads on the encoder shaft from deforming the disk which leads to errors in encoder accuracy. Modular encoders, which are a special type of conventional encoder, also have this feature, but their accuracies are limited by the alignment attainable during installation of the device. The present invention is far less sensitive to these alignments provided certain conditions are met at assembly which shall be discussed in more detail in what follows. The possibility of loss of view of the track is far less of a concern as well because the steering optics can accommodate large excursions of the track.
This invention is an adaptation of Compact Disc (CD) or Laser Optical Disk (LOD) technology and consequently affords some features that conventional encoders cannot provide. Since it is based on this technology, the device takes advantage of the high bit density achievable on CD or LOD which is equivalent to having available extremely fine line widths. This is significant because the closer together one can pack these lines, the greater the achievable resolution of angular motion at a particular radius. The present device should be easily adaptable to other types of mass storage media, such as magnetic disks, magneto-optic disks and electron trapping optical disks. The technique is, thus, adaptable to the media offering the greatest bit density at any point in the evolution of mass storage devices.
A particular advantage of using the laser optical disk and the associated optics incorporated in the present invention is that the laser optical disk is unaffected by normal surface deterioration since the surface which is being optically read is sandwiched between a protective coating and a thick substrate material. Accordingly, the encoder described herein will be less sensitive to dust and contaminants than are high resolution optical encoders of conventional design which employ surface plated disks.